Stroboscopic light source



April 9, 1957 R. B. MaCFARLANE 2,788,462

STROBOSCOPIC LIGHT SOURCE Filed June '7, 1954 INVENTOR.

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STROBOSCGPIC LIGHT SOURCE Richard B. MacFarlane, Oakland, Calif.Application June 7, 1954, Serial No. 434,816

3 Claims. (Cl. 313-205) pressure, the preferred content being sometimesnitrogen but more usually one of the monatornic gases such as helium,neon, or argon, or mercury vapor. Mounted within the envelop and sealedat one end to the wall thereof, is an elongated discharge tube which isusually more or less convoluted. The end of the tube which is not sealedto the wall is open and communicates directly with the rarified gas inthe body of the envelop. Within that end of the tube which is sealed tothe envelop wall there is mounted an electrode (the cathode) which has aterminal projecting outwardly through the wall of the envelop. Withinthe body and external to the tube, the anode st ucture is mounted, suchanode structure some times comprising a sin le electrode, sometimes apair. Terminals for the anode or anodes are brought out through the wallof the tube.

The light source of the present invention is very similar in appearanceto that just described. There are, however, highly important structuraldiflferences which lead to ditlerences in operation of an importantnature which, for certain purposes at least, give the device of thisinvention increased value. Like the prior art devices, the presentinvention comprises a partial evacuated envelop containing a gas orvapor at low pressure. Within the envelop is an elongated,light-permeable discharge tube, preferably of convoluted form, but incontrast with the prior art structures both ends of this elongated tubeare sealed to the wall of the envelop, and electrodes are sealed throughthe envelop wall so as to project into both ends of the tube instead ofone end only; unless the device is to be combined with other apparatusnot directly related to the present invention, no electrode is used inthe body of the envelop, external to the discharge tube itself. At somelocation along the course of the discharge tube, and preferably at alocation within the envelop which is most remote from the electrodessealed therethrough, there is formed an aperture connecting the bore ofthe discharge tube with the body of the envelop.

Among the objects of the invention as embodied in the construction thusdescribed, is to provide a source of higher luminosity for a given powerinput than has heretofore been available; to provide a device of morerigid construction and less liable to breakage; to provide a lightsource of the positive-column type having a lower deionization time thanprior art devices of the same general character and hence able toresolve into separate light flashes pulses recurring at a more rapidrate; to provide a light source of the general character describedwherein the clean up effect or reduction of gas pressure within theenvelop due to adsorption by the walls is reduced, so that the pressurein the envelop remains within the working range for a longer period andthe source therefore has a longer life; to provide a light sourcewherein no deposit a volatilized material from the electrodesupon theenvelop wall can occur and wherein any such deposit occurring upon thewalls of the discharge tube itself'is limitedto the areaimmediatelysurrounding the electrodes and does not extend to portions ofthe device where it can intercept and absorb any material portion of theemitted light energy, thus, again, increasing the effective life of thedevice; to provide a light source which does not have to be poled in aparticular direction with respect to the exciting pulses appliedthereto, so that it may be used in standard types of sockets, either ofits electrodes being equally effective as either anode or cathode; toprovide a light source of the type described wherein only that portionof the contained gas which is activated to emit light is ionized, theremaining gas being relatively cool and providing a large reservoirto'maintain the life of the device as clean-up does occur; and, broadly,to provide a light source of unusually high intensity which is, at thetime, resistant to breakage and uniform in characteristics over longeffective life.

The nature of the invention will be best understood by the followingdetailed description of a specific embodiment of the invention taken inconnection with the accompanying drawing.

The single figure of the drawing is an elevation, partly in section, ofone embodiment of the invention as specifically described herein,portions of the envelop and the discharge tube being broken away inorder to show more clearly the construction.

As shown in the figure, the body of the lamp comprises a completelysealed envelop 1, formed preferably of glass, although for specialpurposes it could be of fused quartz or of other vitreous, lightpermeable material, including in the latter term both completelytransparent and translucent materials. In general, the completelytransparent types of envelop are to be preferred, since maximumillumination is ordinarily desired. The shape of the envelop is of veryminor importance; a cylindrical envelop with domed ends such as isillustrated is convenient, but pear-shaped, substantially globular, orother shaped enelopes may be used if desired.

Mounted within the envelop and sealed to the walls thereof is thedischarge tube 3. The discharge tube is preferably of the same materialas .the envelop itself. In the lamp shown, the discharge tube is shownas convoluted so as to concentrate a relatively long length in arelatively confined space, but it will be recognized that if desiredboth envelop and discharge tube could be r linear, so as to give a linesource of light instead of a concentrated one.

In the present instance both ends of the convoluted discharge tube aresealed into the end of the envelop to which a base 5 of the tube(indicated in dotted lines) is eventually attached. It is alsoconvenient to locate the exhaust tip I through which the lampis-evacuated at this same end of the lamp, where it will be concealedand protected by the base.

Althoughthe dimensions of the lamp are not critical, or, in fact,actually germane to the invention as such, it may be notedthat theparticular lamp here described en1 ploys an envelop 1% inches inoutsidediameter and approximately 2%. inches in total length. The dischargetube within this envelop has a total length of approxi' mately one foot,with a bore or lumen of 4 millimeters, although discharge-tubes varyingfrom 3 millimeters up ward to 20 millimeters in bore may be used, commonpractice being usually within the range of 3 to 7 millimeters.

The particular feature to be noted about this discharge tube is theaperture 9 in its wall, which communicates from the discharge tube boreinto the body of the envelop. While the location of the aperture 9 isnot critical, best results are obtained if its location is spacedphysically as far from the ends of the tube as space will permit.

Electrodes 11 and 11 are sealed through the wall of the envelop at thepoint where the discharge tube joins the latter, so that the electrodesproject into the discharge tube. The material of the electrodes isnotimportant; they may be of iron, or of nickel steel having acomposition which will provide substantially the same coefficient of theglass used, in accordance with well known practice. The electrodes areprovided with terminals 13, 13', for connection with pins 15, 15 securedto the base 5.

In manufacture the envelope is first thoroughly evacuated through thetip 7, before the latter is sealed off. In the course of the evacuationthe envelop and the discharge tube within it are thoroughly out-gased inaccordance with well known practice, so as to prevent contamination ofthe gas filling of the tube by undesired impurities, released from thewalls of the tube when the latter heat in operation. After evacuationand out-gasing whatever gas is to be used as the light source isintroduced at proper pressure and the tip 7 is sealed off.

It may be noted that if desired the final clean-up of the envelop,before the admission of the active gas, may be facilitated-by the use ofone of the well known getter materials. Because such materials usuallyleave an opaque coating on at least a portion of the envelope, however,it is preferred to accomplish the evavuation by a straight pumping andbaking technique. This is a matter of detail, however, and eithertechnique falls within the scope of the present invention.

A wide variety of gases may be used within the lamp. The rare, monatomicgases may be employed, with or without the addition of a small amount ofmercury, the latter, of course, providing a mercury vapor whichcontributes to and usually dominates the spectrum of the light emitted.Of the rare gases xenon gives the most effective light for photographicpurposes if used alone. The mercury spectrum is, of course, highlyeffective photographically, particularly if an envelop or discharge tubetransparent to the ultraviolet is used. Xenon, however, is relativelyexpensive. A light which is very brilliant and quite comparable bothvisually and photographically to that emitted by xenon is provided by amixture of 15% neon and 85% of carbon dioxide. This mixture emits alight having a color temperature of from 6500 to 7000 K. Nitrogen isalso a possible gas for use in tubes of this character, although formost purposes its spectrum is not as desirable as some of the othermaterials mentioned.

With any of the gases mentioned, the amount admitted is preferably suchas to provide a pressure within the envelop from 1 /2 to 6 millimetersof mercury. For discharge tubes of internal diameters within the rangehere described pressures of from 4 to 6 millimeters of mercury are to bepreferred, but it should be noted that with discharge tubes of largerbore it is possible upon occasion to use pressures as high as 20millimeters of mercury. Since, as in all devices of this character,clean-up does occur to some extent, it is desirable that the pressureused be toward the upper limit of the range appropriate to the dischargetube of a given bore, this being conducive to a longer life lamp. The1.5 millimeter figure first given marks approximately the lower limit ofthe practical range, although with suitable exciting voltages it is notnecessarily an absolute limit.

In operation lamps built and described as illustrated have been found toemit approximately 3 times the light, for a given power input, of lampshaving the same length and caliber of discharge tube and the same gascontent,

' adsorption is small.

but of the open-end discharge tube construction which is described inthe prior art. The lamps of this invention have no rectifyingcharacteristics, the two electrodes being substantially identic'aL-andoperating equally Well as either cathode or anode. The structure whereinboth ends of the discharge tube are sealed to the envelop is somewhatmore difiicult to construct than the single sealed-end end type. Thestructure is, obviously, more rigid; where a lamp of the usual open endconstruction is rapidly pulsed it is possible for the free end to set upa suflicient vibratory motion so that it actually can be heard. Thedouble sealed end is much more rigid and resistant to shocks which canocur both in shipment and in operation.

When the tube is excited the entire discharge path lies within thedischarge tube itself; even at the orifice 9 into the main reservoirthere is no apparent luminosity which escapes. The gas which remains inthe envelop, un-

ionized and unatfected by the discharge, is available to cool thedischarge tube and this tends to speed up deionization.

Electrode sputter is limited to the area immediately surrounding theelectrodes and does not discolor the discharge tube to cause absorptionof the emitted light. Absorption of gas by the walls of the dischargetube is also limited very largely to this area, and since this is alsothe highest temperature portion of the tube there is also somereliberation of the absorbed gases, so that the total Outside of thedischarge tube, in the body of the envelop, there is available arelatively large volume of gas which tends to maintain the pressurewithin the discharge tube and prolong the life of the device. Incombination with the rigid construction which tends to preventaccidental termination of the life of the lamp the over-all result is adevice of high efficiency, high constancy of performancecharacteristics, ruggedness and long life.

What is claimed is as follows:

1. A gaseous discharge lamp comprising a partially evacuated envelop, acontinuous elongated light-permeable tube within said envelop and sealedthereto at both ends, a pair of electrodes, sealed respectively into theends of said tube and connections sealed outwardly through said envelopeach consisting of a single conductor connected to one of saidelectrodes, said tube having an aperture formed therein connecting thebore thereof with the body of gas within said envelop.

2. A gaseous discharge lamp comprising a partially evacuated envelop, acontinuous elongated light-permeable tube within said envelop and sealedthereto at both ends, a pair of electrodes, sealed respectively into theends of said tube and connections sealed outwardly through said envelopeach consisting of a single conductor connected to one of saidelectrodes, said tube having an aperture formed therein at a locationspaced from both of said electrodes.

3. A stroboscopic light source comprising a partially evacuated envelop,a continuous convoluted elongated transparent tube within said envelopand having its ends sealed to the wall thereof, a pair of similarelectrodes projecting into the ends of said tube and connections sealedthrough the wall of said envelop each consisting of a single conductorconnected to one of said electrodes, said tube having an aperture formedtherein connecting the bore thereof with the body of gas within saidenvelop at substantially the location most remote from said electrodes.

References Cited in the file of this patent 2,273,520 Henninger et al.u,- Feb, 17,

